The problem of restoring useful vision to a human eye after its cataractus natural lens has been removed has been with us since the introduction of cataract surgery. The solution to this problem has included the use of spectacle lenses, contact lenses, and permanent implantation into the eye of a man-made lens, i.e., an intraocular lens.
Since 1949, when the first implant of an intraocular lens was made, hundreds of thousands of persons have had such implants. Recent advances in cataract surgery have now made the intraocular lens implant procedure a safer and more popular alternative. For example, it is estimated that nearly 40 percent of the people now undergoing cataract surgery select a lens implant, i.e., an intraocular lens, instead of wearing contact lenses or thick cataract-type spectacles.
Although silicate glass was initially considered for use in intraocular lenses, generally such lenses are now made of polymethylmethacrylate (PMMA). PMMA is a polymer formed by polymerization of methyl methacrylate monomer (MMA monomer).
In the aphakic eye, the cornea and crystalline lens work together to absorb ultraviolet radiation or light. The cornea absorbs virtually all of the ultraviolet radiation below 300 nanometers (nm) that reaches the eye (almost all ultraviolet radiation below 286 nm is absorbed by the ozone layer in the earth's atmosphere). Although the cornea also absorbs a portion of the ultraviolet radiation between 300 nm and 400 nm, the natural crystalline lens absorbs the major portion of the ultraviolet radiation in this range.
Thus, when the natural crystalline lens is removed from the eye, ultraviolet radiation that would otherwise have been absorbed by the natural lens can reach the retina.
Recent evidence indicates that the retina can be damaged by ultraviolet radiation. For example, ultraviolet radiation is considered by many opthamologists as a casual factor in erythropsia ("pink vision"), cystoid macular edema, and macular degeneration.
It is thought that retinal damage by ultraviolet radiation results from absorption of light energy and conversion of this energy into heat within the cells. Damage to the retina can also result from molecular absorption of ultraviolet radiation which can place a cellular molecular species in an excited state, resulting in various chemical reactions of the excited species.
It appears that damage to the eye from ultraviolet radiation is cumulative. Thus, it is very important that an individual who has his natural crystalline lens removed take precautions to shield the retina of his eye from ultraviolet radiation.
Studies have also provided supporting evidence to show that ultraviolet radiation is a causal factor in polymer degradation of polymethylmethacrylate, the material from which intraocular lenses can be made. Thus, it is also important that an implanted intraocular lens be protected from ultraviolet radiation.
Since polymethylmethacrylate does not absorb any significant portion of the ultraviolet radiation in the 300-400 nm range, persons who have had a polymethylmethacrylate intraocular lens implant have been prone to retinal damage from ultraviolet radiation or have had to protect their pseudophakic eye by using spectacle lenses having ultraviolet light absorbing capability.
It is important, therefore, that an intraocular lens be provided that has ultraviolet absorbing capabilities, firstly to protect the retina of the eye from radiation damage, and secondly to retard damage to the intraocular lens itself from such ultraviolet radiation. It is desired that the ultraviolet absorbing intraocular lens have absorbing capabilities which approximate the capabilities of the natural crystalline lens which has been removed. Further, the ultraviolet absorbing intraocular lens must be biocompatible with body fluids and tissue and have desired optical properties.